1. Field of the Invention
This invention generally relates to offshore structures for use in arctic regions and more particularly to structures which offer protection against the dynamic forces of ice sheets and other ice masses. 2. Description of the Prior Art
To meet the increasing demand for oil and gas, exploration and production of petroleum products has been extended to offshore locations which have hostile weather conditions during much of the year. Among these locations are the bodies of water located in the arctic regions of the world such as northern Alaska, Canada and Greenland. One of the major problems encountered in offshore arctic regions is the continuous formation of sheets of ice which can be as much as 8 feet thick. These ice sheets are not stationary. Under the influence of winds and sea currents, they move laterally through the water at rates of up to several hundred feet per day. Such dynamic masses of ice can exert enormous crushing forces on anything in their path. Therefore, any offshore structure which is to operate in an arctic environment must be able to withstand or overcome the dynamic forces created by moving ice.
Another danger encountered in arctic waters are pressure ridges of ice. These are huge mounds of ice which usually form within ice sheets and which may consist of snow, pack ice and overlapping layers of sheet ice. Pressure ridges can be up to 100 feet thick and can, therefore, exert proportionately greater force than ordinary sheet ice. The capacity of pressure ridges for causing the catastrophic failure of an offshore structure is very great.
Bottom supported stationary structures are particularly vulnerable in offshore arctic regions, especially in areas of deep water. All of the force of the ice sheet or pressure ridge is directed near the surface of the water. If the offshore structure comprises a drilling platform supported by a long, comparatively slender column which extends well below the surface, the bending moments caused by the laterally moving ice may well be sufficient to crush or buckle the platform.
One approach to the above problem, which has been suggested by Gerwick and Lloyd (1970 Offshore Technology Conference), comprises a bottom supported, inverted conically shaped structure. The moving ice strikes the slanted wall of the cone shaped structure and is uplifted. The uplift of the ice not only tends to break the ice, but also substantially alleviates the horizontal crushing force of the ice on the structure. However, if water depth is great (in excess of 200 feet), such a structure might be prohibitively expensive to build because the inverted conical shape would require a very substantial volume of the total hull to be below the surface of the water. Another approach, disclosed in U.S. Pat. No. 3,766,874, is a floating conical structure. Such a structure employs a hull moored to the sea bottom and having a frusto-conical shape to fracture ice impinging on the hull. Since the structure floats, it is capable of operating in deeper waters. Both of the above structures however, are designed to alleviate the crushing forces of the ice by virtue of their geometric shape. They do not possess any active ice breaking capability. Both the bottom founded platform and the moored floating structure are fairly rigid structures which cannot yield to or counter the stresses of the moving ice.
Several external ice protection systems have been proposed which actively attack the ice mass by either melting, diverting or breaking the ice. A typical protection system is described in U.S. Pat. No. 3,807,179 which discloses an apparatus in which ice lifting elements are supported around the columns or legs of an offshore platform. Means are provided for moving the elements upwardly to break and lift the ice sheet as it moves toward the structure. Another type of apparatus is described in U.S. Pat. No. 3,759,046 which discloses the use of heat transfer devices disposed along each portion of the platform legs extending through the surface of the water. The heat transfer devices warm the ice adjacent the platform legs to within about 1.degree. or 2.degree. C. of its melting point so as to lower the strength of the ice sufficiently to permit easier breakage.
While the external systems, such as those proposed above, afford some protection against ice sheets and pressure ridges, these systems are complicated and costly and will not protect the offshore structure against extreme forces which would otherwise result in the catastrophic failure of the structure. Accordingly, in the area of offshore structures, the art has lacked a structure or system which is well adapted to an arctic environment and which is capable of withstanding the extreme forces caused by dynamic masses of ice.